1. Field of the Invention
The present invention relates to a Sn-coated copper alloy strip used as a conductive material for connecting parts such as a terminal in the field of automobiles and other consumer products, which can maintain low contact resistance at a terminal contact portion for long time.
2. Description of the Related Art
In automobiles electric equipment, mating connectors comprised of male and female terminal are used for connecting wire harnesses. Recently, electronic equipment is also installed in engine room of automobiles, and connectors are required to keep good electrical property (low contact resistance) for long time at high temperature.
Long time exposure at high temperature of a Sn-coated copper alloy strip increases contact resistance of the strip, because Cu and alloying elements in copper alloy strip diffuse to the surface of the tin coating layer and are oxidized. As a countermeasure, copper alloy strip with three coating layers-base layer of Ni, etc., intermediate layer of Cu—Sn intermetallic compound, and outermost layer of Sn— is suggested in JP-A No. 2004-68026. By this structure, the Ni plating layer prevents diffusion of Cu or other alloy elements from the copper alloy matrix, the Cu—Sn intermetallic compound layer suppresses diffusion of Ni from Ni plating layer, and retains low contact resistance long at high temperature. JP-A No. 2006-183068 describes a Sn-coated copper alloy strip in which surface of the copper alloy strip is roughened, and three layered structure above mentioned is applied as a coating layer on it. Further, JP-A No. 2010-168598 describes a Sn-coated copper alloy strip with three layered structure above mentioned but in which Cu—Sn intermetallic compound layer is of two layers, a lower ε(Cu3Sn) layer next to the Ni coating layer with the coverage area rate over the Ni layer is 60% or more, and upper η (Cu6Sn5) layer beneath the Sn plating layer. With this structure, contact resistance after long period at high temperature is stabilized, and exfoliation of the plating layers is prevented.